Electrophysiological observations on the teleost olfactory bulb

Abstract

Previous electrophysiological research on the fish olfactory system is reviewed and the conclusion reached that present knowledge concerning, in particular, the fundamental physiology of neuronal connections within the olfactory bulb, was rudimentary and confused. Field potentials were evoked in the olfactory bulb of teleost fish by electrical stimulation of the olfactory tract and nerve. The potential wave recorded at the bulbar surface consists of four components, N1, N2, N3, AND P, all of which appear to be of post-synaptic origin when the nerve is stimulated, whence they are usually preceded by a triphasic potential thought to represent the compound action potential of olfactory nerve fibres. The N1 wave evoked by olfactory tract stimulated is not of synaptic origin. It probably represents the synchronous antidromic activation of secondary neurons. The waves analysed with respect to voltage and time related to the underlying histology. The results indicate that the extracellular current flow around bulbar neuronal elements is essentially similar to that already described for mammals and is probably generated by similar pathways. This is surprising in view of fundamental anatomical dissimilarities, particularly regarding the dendritic field of mitral cells. The field potentials proved to be useful in the identification of single units at the time of recording. The spontaneous and evoked activity of identified mitral and granule cells could often be inhibited by stimulation of either the nerve or tract. The evoked field potentials could usually be similarly inhibited. Evidence has been obtained that this inhibition is mediated GABA and that it may well take place via a recurrent pathway involving reciprocal dendrodendritic synapses as in the mammalian system. Evidence was also obtained that this inhibition may, in part, result from the activation of granule cells by andrenergic centrifugal fibres when the olfactory tract is stimulated. Natural chemical stimulation of the olfactory mucosa with amino acid solutions produced a complex pattern of responses. Each odorant normally produced a unique pattern of excitatory and inhibitory responses across all units. Chi-square values were calculated for stimulatory effectiveness between forty-five pairs of odours. L-serine and L-alanine consistently showed a high degree of similarity with several other odours. The converse was true for GABA and L-histidine, although this pair had a high chi-square value when mutually compared. Enantiomeric pairs of amino acids were often found to have opposite stimulatory effects on bulbar units. These results are discussed in relation to the possible properties and configurations of odorant receptor sites for amino acids in the fish olfactory mucosa.